This invention relates to optical communications, and in particular to a method of optical domain clock signal recovery from high-speed data, which is independent of the data format or the optical signal rate.
Optical fiber networks are in widespread use due to their ability to support high bandwidth connections. The bandwidth of optical fibers runs into gigabits and even terabits. Optical links can thus carry hundreds of thousands of communications channels multiplexed together.
One of the fundamental requirements of nodal network elements in optical networks is the capability to extract the line rate clock from the incoming signal. Presently, this is achieved by converting the incoming optical signal into an electrical signal followed by clock extraction using an application specific electronic circuit. As optical networks become increasingly transparent, there is a need to recover the line rate clock from the signal without resorting to Optical-to-Electrical, or O-E-O, conversion of the signal.
Future optical networking line systems will incorporate service signals at both 10 Gb/s, 40 Gb/s and much higher data rates, along with the associated Forward Error Corrected (FEC) line rate at each nominal bit rate. The FEC rates associated with, for example, 10 Gb/s optical signal transport include the 64/63 coding for 10 Gb/s Ethernet, the 15/14 encoding of SONET-OC192 FEC, and the strong-FEC rate of 12.25 Gb/s. As these networks tend towards optical transparency, the nodal devices in the optical network must work with any commercially desired line rate, independent of format, whatever that is or may be. Thus, one of the fundamental functions these devices must provide is the capability to extract the clock from an arbitrary optical signal. Moreover, to maintain the high speeds of modern and future data networks, as well as increase efficiency, this clock recovery must be done completely in the optical domain.
In future All Optical Networks (AON) the same network element will need to handle both 10 Gb/s and 40 Gb/s. Consequently, the clock recovery in these network elements must be tunable over a wide range of frequencies.
Previous embodiments of clock recovery systems are experimental in nature, and relegated to research laboratories. They do not include the possibility of recovering the line rate clock from the various ubiquitous NRZ data formats. Additionally, any tuning of the clock signal is done using a linear phase section.
What is therefore needed is an all optical clock recovery system that can operate upon any given optical signal, regardless of its format or bit rate. What is further required is a system that exploits non-linear optical elements to reshape the clock output for optimal retiming of the various data formats.
A method and circuit are disclosed for the recovery of the clock signal from an arbitrary optical data signal. The method involves two stages. The first stage consists of a Semiconductor Optical Amplifierxe2x80x94Asymmetric Mach-Zehnder Interferometer, or SOA-AMZI, preprocessor, which is responsible for transforming an incoming NRZ type signal into a pseudo return to zero (xe2x80x9cPRZxe2x80x9d) type signal, which has a significant spectral component at the inherent clock rate.
This preprocessing stage is followed by a second stage clock recovery circuit. In a preferred embodiment the second stage is implemented via an SOA-MZI circuit (symmetric in structure, i.e., no phase delay introduction in one of the arms) terminated by two Distributed Feedback (DFB) lasers that go into mutual oscillations triggered by the dominant frequency of the first stage""s output signal. The SOA-MZI is tuned to adjust the input phase of the oscillatory signal into the DFBs. This provides the tuning and control of the oscillation frequency of the output clock signal. The SOA gain currents can be adjusted to reshape the clock signal, which is the output of the second stage.